Implement getShakeExtra

[bgamari]

This is a convenient utility to extract a particular key from shakeExtra.
Unfortunately we can't make this interface typesafe since shakeExtra is a bit
too general: there is no known relationship between the key TypeRep and the
value at that key.

[ndmitchell]

I think the right thing to do is to add a corresponding addShakeExtra which does the insert correctly, and also at startup check all items in shakeExtra obey the types/values invariant. I'll add those changes on top of this.

[ndmitchell]

I added addShakeExtra and made it an extra for shakeExtra to not obey the obvious invariant - meaning the failure in getShakeExtra is now impossible to hit.